The commonly used electro-optical shutter for high quality electro-stereoscopic applications is the surface mode or .pi.-cell LC device. It is preferred because it has superior speed, particularly for the transition from its occluded to transmissive state. However, there are stereoscopic display applications for which the slower twisted nematic liquid crystal ("TN LC" or "TN") device is adequate; for some products the speed of the surface mode device may not be required. For example, for a video toy or game, in which a flickering 30 field/second/eye system is employed, the use of a slower shutter results in a reduction in brightness, which may be of help reducing the user's perception of flicker. In addition, there are means for improving the speed of the TN device, a description of which is beyond the scope of this disclosure. One possible approach may be found in Bos, U.S. Pat. No. 4,566,758. However, we have not tested the devices described in U.S. Pat. No. 4,566,758 in a stereoscopic application.
The present disclosure is concerned with methods for the elimination of TN artifacts which produce an unpleasant visual effect for the user. The term "artifact" may not be entirely appropriate, since these effects are an intrinsic part of the electro-optical characteristics and performance of the TN device. Nevertheless, we will continue to call them artifacts, since they are undesirable for the application contemplated here; as a shutter in a stereoscopic selection device. These artifacts are "stumble" and "bounce", which, respectively, produce a flickering effect when the selection device, hence the shutter, is turned on and off.
Electronic stereoscopic displays have gained acceptance to a great extent because of the success of the CrystalEyes.RTM. product described in U.S. Pat. No. 4,884,876 and U.S. Pat. No. 4,967,268 by Lipton et al. '876 describes a high performance LC shutter with means for driving it in a power efficient manner. '268 describes an infrared (IR) communications link to synchronize the eyewear's shutters with the video refresh rate. Another reason for the growing acceptance of stereoscopic electronic imaging is described in Lipton et al. U.S. Pat. No. 4,523,226 which provides the basis for producing a flickerless stereoscopic display; the technique is used by many manufacturers of computer graphics workstations and PC graphics boards.
FIG. 1 shows the set up for using the CrystalEyes.RTM. product. An IR emitter is located on the display monitor. The left/right control signal from the electronic imaging source, typically a "stereo-ready" graphics computer (not shown), is fed to the emitter which broadcasts IR signals (dotted lines) containing encoded sync information. This information is received by the CrystalEyes.RTM. eyewear worn by the user and is used to synchronize the eyewear's shutters to the video field rate. The appropriate eye will then see its appropriate perspective viewpoint, and not the unwanted view, and the result will be the perception of a stereoscopic image.
FIG. 2 shows a preferred embodiment of the CrystalEyes.RTM. eyewear. The eyewear is powered by on-board lithium batteries which provide electric energy for a flex circuit (which incorporates a custom integrated circuit) which is used to drive the LC shutters (lenses). An IR sensor located in the front of the eyewear sees the IR signal broadcast by the emitter and uses this information to open and close the LC shutters in sync with the video field rate.
The LC lenses which are employed in the CrystalEyes.RTM. product as described in the '876 patent are preferably surface mode cells, which have good performance for stereoscopic application, but these are not the only kind of LC shutter which may be used. A viable alternative for certain applications, as stated above, is the TN device.
The TN device suffers from two drawbacks. As stated above, one phenomenon we call stumble, and the other is called bounce. In the case of stumble, when the eyewear is turned on, the shutters flicker for a moment, usually less than a second. In the case of bounce, when the eyewear is turned off, there is flicker. A clean transition from on to off or off to on, in which there is no flickering, is greatly to be preferred. This transition of the shutter in periodic bursts, in which there is unwanted modulation of light, is disturbing to the user.
Bounce has been discussed in the literature, in particular, in Bos et al., "A Liquid-Crystal Optical-Switching Device (.pi. Cell)," SID Digest, 1983, pp. 30-31.
Concerns about stumble and bounce are raised by the need for the CrystalEyes.RTM. product (using surface mode shutters) to incorporate means for keeping the shutters functioning when the eyewear is turned away from the IR emitter. In this case the IR sensor in the eyewear cannot see the IR signal (and thus, cannot be maintained in its normal shuttering mode by the IR signal). However, even if the IR sensor cannot see the IR signal, the shutters must continue to operate as shutters to prevent their falling into the rest state. Unlike the TN shutter, which has two states (open and closed) the surface mode device has a third state which occurs a few seconds after the shutter ceases to be powered. More information about this characteristic of the surface mode device may be found in aforementioned article by Bos et al.
In brief, surface mode shutters operate through a birefringent effect which persists when voltage is not applied to the device. This birefringence is responsible for coloration of the shutter. The effect is well known and may be illustrated by crumpling cellophane (which is birefringent) and placing it between two sheets of linear polarizer with crossed axes. TN cells on the other hand, operate according to the principal of optical activity, and the spiral orientation of the molecules, necessary for the toggling of the axis of polarization to achieve transmission, needs no voltage.
In the rest state, the surface mode shutter becomes colored--depending upon the thickness and type of the nematic liquid it may (for example) appear green, purple, or yellow. Such coloration is obtrusive to the user and it is necessary to keep the surface mode shutter shuttering by means of an internal oscillator, so that it will have a neutral transmission.
An alternative mode is to hold the surface mode shutter in the open state, which is needed for users looking at documents, for making notes, or looking at monitors not operating in the stereoscopic mode. This open state can be achieved by applying the square wave waveform shown in FIG. 5 to the shutter (at times when the shutter does not receive the IR control signal intended to maintain it in its normal shuttering mode). By providing the FIG. 5 signal with a peak-to-peak voltage of two to eight volts, the surface mode shutter will remain in its transmissive mode.
The concern of the present invention is the operation of TN devices. The specification will disclose means to successfully employ a TN device. As mentioned, the TN device has two states; it is in one such state when a voltage is applied (the closed state), and the other when no voltage is applied (the open state). FIG. 3 shows the typical construction of a conventional TN shutter, with the TN LC cell sandwiched between two linear sheet polarizers whose axes (PA1 and PA2) are orthogonal. The axes of the facing inside surfaces of the cell are rubbed orthogonally and the sheet polarizer juxtaposed with each glass surface of the cell has its axis aligned parallel with the rub direction of the juxtaposed piece of glass.
Given such construction, the TN device will be transmissive (open) with no voltage applied across the LC cell, and closed when voltage source V (shown in FIG. 3) applies a voltage across it. As an alternative to the continued shuttering of the TN LC device (under control of an oscillator) at times when no IR sensor in the eyewear receives an IR control signal (for maintaining the TN shutter in a normal shuttering mode), the power could be turned off to the TN shutter in the absence of the IR control signal, to cause the shutter to make a transition to a stable open state. However, if this is done the TN exhibits stumble and bounce.
If the shutter of a conventional TN device (mounted in eyewear) is turned off when a user wearing the eyewear turns away from an IR control signal emitter, the user will experience bounce or flicker annoyingly for a moment. When the shutter is turned on again (when the user faces the IR control signal emitter), it will momentarily stumble or flicker distractingly, before it resumes the normal shuttering mode. The invention provides a means for the elimination of these undesirable effects so that the function of the selection device will be transparent to the user.